In recent years, a polyorganosiloxane has attracted attentions as a material having good light permeability, good heat resistance, low gas permeability and good chemical stability. Polyorganosiloxanes having various properties are provided by changing a type of siloxane monomers, composition of raw materials and reaction conditions in the production process. On account of this, polyorganosiloxanes have been put to practical use in the various fields.
The organopolysiloxane is generally prepared by hydrolysis and condensation reaction which are caused by contacting a chlorosilane and/or alkoxysilane with a stoichiometric amount of water in an organic solvent and in the presence of an acid or base catalyst. However, in this method, a significant amount of silanol groups remains often in the obtained polyorganosiloxane and react between each other in storage to increase the viscosity to cause a problem in storage stability. Further, the unstable silanol group remaining in the polymer may cause cracks and decrease of the adhesiveness in a long-term use. Further, because the polyorganosiloxane obtained from the hydrolysis and condensation reaction has a random structure, polyorganosiloxane with desired properties is not always obtained.
Examples of the other methods for preparing a polyorganosiloxane include a method where an organic silicon compound having a silanol group, i.e., —SiOH, condensation reacts with each other; a method where an organic silicon compound having a silanol group, i.e., —SiOH, condensation reacts with an organic silicon compound having an alkoxysilyl group, i.e., —SiOR; and a method where an organic silicon compound having a alkoxysilyl group, i.e., —SiOR, condensation reacts with each other, wherein R represents an alkyl group or an alkoxyalkyl group. In the aforesaid condensation reactions, an amount of a silanol group remaining in the polyorganosiloxane obtained is small. However, these methods need chemically fierce catalysts to cause condensation reaction, for instance, strong acids such as sulfuric acid and hydrochloric acid; strong bases such as sodium hydroxide, potassium hydroxide and tetramethylammonium hydroxide; and Lewis acids. When the aforesaid catalysts are used, siloxane bonds (Si—O—Si) are cut to cause rearrangement during the reaction and, therefore, the polyorganosiloxane obtained has a random structure.
Japanese Patent Application Laid-Open No. H02-235933 describes that a borate or phosphate of sodium or potassium is used as a catalyst and a silanol-containing siloxanes is subjected to a condensation reaction in the presence of the catalyst to prepare an organosilicone condensate. Japanese Patent Application Laid-Open No. H03-197486 describes that silanol-containing siloxanes is subjected to a condensation reaction in the presence of a catalyst selected from the group consisting hydroxide, chloride, oxide and basic metal salt of an alkali metal or alkaline earth metal to prepare a polyorganosiloxane. Japanese National Phase Publication No. 2006-508216 describes that hydroxide of magnesium or calcium can work as a catalyst in condition of the presence of a protonic solvent to promote a condensation reaction between a silanol-containing siloxane and an alkoxysilane. Japanese National Phase Publication No. 2010-506982 describes that a silicon containing compound having a silanol group and/or an alkoxysilyl group reacts in the presence of a catalyst selected from the group consisting of strontium oxide, barium oxide, strontium hydroxide, barium hydroxide and a mixture thereof to prepare an organosilicone condensate.
In the methods described in the afore-mentioned patent literatures, rearrangement of the polyorganosiloxane chain is minimized and, thus, a polyorganosiloxane having a controlled structure is obtained. Further, these methods have an advantage that the catalyst can be easily separated from the obtained polyorganosiloxane by filtration because these catalysts are solid. These advantages are favorable particularly in fields where accurate control on materials is required and no remaining impurity is tolerable, for instance, the fields of optical materials, electronic materials and medical materials.